3 5 8C H A P T E R 1 8 ■ L e g a l R e a s o n i n grequired a permit from the Board of Supervisors for any public laundry not
operated in a brick or stone building. On its face, this ordinance was sup-
posed simply to prevent fires. In practice, however, the Board of Supervisors
granted permits to all but one of the non-Chinese applicants and denied per-
mits to all the Chinese applicants. Because of this practice, Yick Wo claimed
that the ordinance violated the equal protection clause. The Supreme Court
agreed and declared the ordinance unconstitutional, at least insofar as it
gave the city power to grant and refuse permits “without regard to the com-
petency of the persons applying, or the propriety of the places selected for
the carrying on of business.”
The argument from a precedent to a decision in a present case is often
presented as an argument from analogy. In this form, the argument empha-
sizes similarities between the cases, and then concludes that the decision in
the present case should be the same as in the precedent. Plessy’s argument
then appears to run something like this:
(1) The ordinance in Yick Wo was declared unconstitutional.
(2) The ordinance in Yick Wo is similar to the statute in Plessy in several
respects.
∴(3) The statute in Plessy also ought to be declared unconstitutional.
This argument is not very good as it stands, so we need to add some sup-
pressed premises.
The first step is to construct a list of the respects in which the cases are
similar. That is not always easy. When we are evaluating someone else’s ar-
gument, we can focus on the similarities that he or she mentions. But when
we are constructing our own legal arguments, we have to be more creative;
we have to formulate the respects in which the cases are supposed to be
similar. Some similarities do not matter: It is clearly irrelevant that the laws
in Yick Wo and Plessy both contain more than ten words or that both apply
to large cities. This much is assumed by both sides in the case, and legal rea-
soning would be impossible without assuming that many such similarities
are irrelevant. Likewise, not all differences matter: It is not important, even
if true, that Yick Wo was married and over fifty years old, but Plessy was
not. To discount or distinguish the precedent, one must show that some dif-
ference between Yick Wo and Plessy is important enough to justify reaching
different decisions in these cases. The central question, then, asks which fac-
tors—similarities and differences—do matter. In general, the answer is that
a factor is relevant when it is needed to justify the decision in the precedent.
These factors are often called the ratio decidendi—the reason for the decision.
Incorporating the doctrine of precedents as a suppressed premise, the ar-
gument from Yick Wo to Plessy can now be reconstructed as follows:
(1) The ordinance in Yick Wo was declared unconstitutional.
(2) The ordinance in Yick Wo is similar to the statute in Plessy in several
respects (A, B, C, D, and so on).97364_ch18_ptg01_351-382.indd 358 15/11/13 11:38 AM
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